FIG. 1 is a schematic block diagram indicating the main functional components of a 3GPP wideband code division multiple access (WCDMA) receiver. Reference numeral 2 denotes an antenna which receives a wireless transmission and supplies it in analog form to RF and IF stages 4. A receiver front end 6 includes the functions of analog to digital conversion and supplies digital samples to a signal detection block 8. The signal detection block 8 can be implemented in a number of ways and is responsible for de-scrambling and de-spreading the received coded signal samples. The signal to interference plus noise ratio or signal to disturbance ratio (SIR) of the received signal can be measured from the output of the signal detection block 8, in an SIR estimation block 9. For each time slot a block is received which comprises a plurality of transport channels (TrCH) multiplexed onto a dedicated physical channel (DPCH in 3GPP WCDMA). As shown in FIG. 1, after signal detection and channel decoding, the decoded data bits are supplied to a Cyclic Redundancy Check (CRC) block 12. The CRC check indicates whether or not the data block has been correctly decoded.
For interference-limited wireless systems, such as those based on CDMA technology, link adaptation is performed by a Transmit Power Control (TPC) mechanism, which ensures that sufficient but not excessive power is transmitted to achieve an adequate received signal quality. In a 3GPP WCDMA system, the power control mechanism comprises two parts: 1) a so-called “outer-loop” algorithm 14 that sets and adjusts a target signal to interference plus noise power ratio (SIR) in order to meet a Block Error Rate (BLER) target set by a network; and 2) a so-called “inner-loop” algorithm 16 that provides fast feedback to the transmitter in order that the transmitter can adjust its transmitted signal power so that the receiver SIR target is met. The inner-loop transmit power control 16 is typically based on the comparison between a target SIR (SIRtarget) and an SIR estimated from the received signal (SIRest). The outer-loop mechanism 14 increases or decreases the SIR target in response to the receipt of block error information, which is typically derived by the pass/fail of the CRC check 12. If a data block is received correctly (CRC pass), then the SIR target is decreased; if a data block is received incorrectly (CRC fail), then the SIR target is increased. In a typical implementation, the amount the SIR target is decreased following a correctly decoded block is equal to some step size (in dB) multiplied by the target block error rate, and the amount the SIR target is increased following an incorrectly decoded block is equal to the step size multiplied by one minus the target block error rate. For example, for a 10% BLER target and a 1 dB step size, the SIR target will be decreased by 1*0.1=0.1 dB following a good block and increased by 1*(1−0.1)=0.9 dB following a bad block. This has the effect that, for typical BLER targets, many more good blocks are required to lower the target than bad blocks to raise it by the same amount. In normal circumstances, the inner-loop power control is able to adjust the transmitted power to meet the new target in a short period (in WCDMA the power can be changed by 1 dB per slot). This “normal case” is illustrated in FIG. 2, which is a graph of the SIR measure (----) and SIR target (—) over time, where time may be measured in number of Transmission Time Intervals (TTIs), number of slots, number of radio frames, or other units. However, under certain conditions, such as when the transmitter has reached its minimum allowed transmit power, it may be the case that the actual SIR estimated at the receiver cannot decrease as low as the target SIR. In that case it is likely that the BLER will be lower than the target rate (it could even be zero) so the SIR target will keep being decreased even though there is no possibility of the actual SIR tracking it, as illustrated in FIG. 3. If conditions then change, for example by the receiver moving further away from the transmitter, the very low SIR target will cause the inner-loop power control to dramatically lower the transmit power, possibly causing the receiver to lose synchronization (“out-of-sync”) with the transmitter with the result that the call could be dropped.
It is an aim of the present invention to obviate or at least mitigate the disadvantages discussed above.